Cardiac muscle Coloured transmission electron micrograph TEM of cardiac muscle fibrils orange and purplefrom a healthy heart Mitochondria blue supply the muscle cells with energy The muscle fibrils or myofibrils are crossed by transverse tubules dark purple lines These tubules mark the division of the myofibrils into contractile units sarcomeres The central structure is a capillary seen in crosssection with a biconcave red blood cell red passing along it Cardiac muscle is under subconscious control and continuously contracts to pump blood around the body without tiring Magnification x when printed centimetres across